Semiconductor image sensors are used to sense radiation such as light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors are widely used in various applications such as digital still camera or mobile phone camera applications. These devices utilize an array of pixel sensors located in a substrate that may include photodiodes and transistors, and that can absorb radiation projected toward the substrate and convert the sensed radiation into electrical signals.
Photodiodes such as complementary metal oxide semiconductors (CMOS) diodes are commonly used for sensing images in cameras and other video or photo devices. Recently, photodiode devices have been improved by using backside illumination (BSI). A BSI image sensor device is one type of image sensor device using a photodiode having a photosensitive region that converts received electromagnetic energy, such as light, into electrical charge. Generally, photolithography processes deposit structures such as gate oxides, metal interconnects, and the like, on the top side of silicon wafer or other substrate. Early photodiodes gathered light from the top, the same side where the device structures were applied. Metal interconnects deposited on the top surface of the photodiode substrate can block portions of the photosensitive regions of the photodiode, degrading the picture quality and individual photodiode sensitivity.
These BSI image sensor devices are operable to detect light projected onto its backside. A BSI image sensor device has a relatively thin silicon substrate (e.g., a few microns thick) in which light-sensing pixels are formed. Ideally, the substrate thickness is reduced so that light may enter the backside of the device and strike the photosensitive region of the photodiode, eliminating obstruction and interference during image capture from deposited structures and metal interconnects. BSI is the collection of light from the backside of the photodiode substrate, with interfering structures such as metal interconnects, gate oxides or the like deposited onto the top side of the substrate, and then the substrate abraded or otherwise thinned to allow light to pass though the substrate and affect the photosensitive region of the photodiode. The quantum efficiency and the full well capacity of BSI image sensors may depend on the size of the radiation-sensing region. Thus, the ability to reduce the amount of interference derived from overlaying metal contacts increases the quantum efficiency of the image sensors by permitting more incident light to strike the photosensitive portions of the image sensor without being blocked by an overlying structure. Additionally, a larger photosensitive region and associated well for each image sensor permits a greater amount of light information to be collected. Therefore, a greater fill factor, or a greater ratio of photodiode area to overall die area, permits greater efficiency in image gathering for a given die size.